For systems applications, solar cells are generally assembled in series strings, which are then grouped in panels. If an individual solar cell in the series-connected string is shadowed, while the remainder of the string is illuminated, the photocurrent must still flow through the shadowed photocell. In this regard it is noted that the output photocurrent from an illuminated solar cell is in the "reverse" direction or the normally non-conducting direction for the solar cell diode when it is not illuminated. When current is forced through a shadowed photocell it may be brought to the reverse breakdown point, often resulting in subsequent degradation in its performance. This problem is especially acute with gallium arsenide solar cells. To protect the cell, a bypass diode with reverse orientation may be wired in parallel with the solar cell diode. When the solar cell is shadowed, the photocurrent from the other cells in the series-connected string flows through the bypass diode in its forward direction, thus protecting the shadowed photocell. Of course, when the solar cell is not shadowed, the bypass diode is back biased and may be ignored, as long as its leakage current is low.
Separate bypass diodes are typically used to accomplish this objective, resulting in often unacceptable increases in cost complexity and weight.
An article describing an arrangement of the type described above is entitled "Solar Arrays with Integral Diodes" by R. M. Diamond and E. D. Steele, and was included in a book entitled, "Solar Cells" edited by J. F. Faugere and D. A. Nutt; Gorden and Breach Science Publshers, Ltd., 12 Bloomsburg Way, London W.C. 1, England, 1971. As described in the foregoing article, the bypass diodes were "integral" in that they were fabricated separately on the back of the silicon solar cell, but they had to be separately wired into the circuit.
Additional articles relating more generally to the technology and methods for making gallium arsenide solar cells, include the following:
1. "Overview of GaAs Solar Cell Production", by Y. C. M. Yeh et al, I.E.C.E.C., April 1984.
2. "Review of Mantech Program For GaAs Solar Cells", by P. A. Iles, Kou-I Chang and W. Pope, presented at I.E.E.E. Photovoltaic Specialists Conference, May 6-10, 1987, New Orleans, LA.
3. "Heterostructure GaAs/Ge Solar Cells", by R. K. Morris, K. I. Chang, et al., presented at I.E.E.E. Photovoltaic Specialists Conference, May 6-10, 1987, New Orleans, LA.
Thus, it has been recognized that it would be desirable to use bypass diodes in solar cell arrays, but the use of separate discrete bypass diodes greatly complicates solar cell panel assembly; and the use of the "integral" diodes requiring separate wiring connections as described in the first article cited above, has much the same disadvantages.
Accordingly, a principal object of the present invention is to provide a monolithic solar cell and bypass diode configuration which avoids the foregoing disadvantages.